1. Field of The Invention
The present invention relates to capacitive transducers and particularly to sensors wherein changes in pressure are transduced into a measurable variation in the capacitance ratio of a pair of capacitors. More specifically, this invention is directed to improved methods for measuring pressure differential and especially to methods for compensating measuring devices of the variable capacitance type for changes in dielectric constant and/or mechanical distortion. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
2. Description of The Prior Art
Sensors wherein a condition being monitored is transformed into a variation in capacitance are well known in the art. While not limited thereto in their utility, such capacitive transducers are particularly well suited for use in monitoring pressure or pressure differences. Examples of prior art capacitive pressure transducers may be seen from U.S. Pat. Nos. 4,295,376, 4,458,537 and 4,562,742.
A class of known capacitive transducers of the "push-pull" type define a pair of capacitors which may utilize a common fluidic dielectric. The capacitors are often concentric and in such case first plates of each capacitor are carried by a diaphragm-like member which will be deflected as the condition being monitored varies. Such deflection will simultaneously change the spacing between the plates of both of the capacitors and thus will change the capacitance thereof. The member on which the movable capacitor plates are supported will, when a pressure differential is to be measured, typically be mounted and driven such that the spacing between the plates of the inner and outer capacitors will vary in opposite directions. The change in the ratio of the capacitance of the two capacitors is measurable electronically and will be commensurate with the change in the pressure differential or other monitored condition.
Previously available transducers of the concentric capacitor type are generally well suited for monitoring the pressure differential between two points in a fluid system such as, for example, a pipe line. However, the previously available devices are known to exhibit variations in the change in the capacitance ratio for reasons other than a change in the condition being monitored. By way of example, a change in the static pressure in a pipe will cause unequal percentage changes in the capacitance of the concentric capacitors. Such unequality will result in the transducer providing an output indicative of a differential pressure change, when in fact the pressure difference has remained the same while the static, i.e., the background, pressure has varied.